1. Field of Invention
The invention relates to an induction heating cooker, and more particularly to an induction heating cooker and a control circuit therefor.
2. Related Art
With the recent progress of the technology, an induction heating cooker has become an indispensable electronic appliance for the modern human beings. The induction heating cooker is the kitchenware for converting the electric energy into the thermal energy according to the electromagnetic induction principle. In the induction heating cooker, a rectification circuit transforms an AC voltage with the frequency of 50/60 Hz into a DC voltage, and then a control circuit transforms the DC voltage into a high-frequency voltage with the frequency ranging from 20 to 40 KHz. The rapidly changing current flows through the coil so that the rapidly changing magnetic field is generated. When the line of magnetic force in the magnetic field passes through the metal pan, especially made of the magnetic-conductive and electro-conductive material, many small vortexes are generated in the bottom metal body so that the pan itself generates heat rapidly to heat the article in the heater.
FIG. 1 is a circuit block diagram showing a conventional induction heating cooker. Referring to FIG. 1, the induction heating cooker includes an inductive coil 10, a capacitor 11, a bridge rectifier 12, an insulated gate bipolar transistor (IGBT) 13 and a control circuit 14, which includes a pulse generator 141, a comparator 142 and firmware 143. The firmware 143 mainly functions to control the pulse generator 141. The user controls the output power and the on/off switch on the control panel of the induction heating cooker to operate the cooker. The firmware 143 controls the operation of the pulse generator 141 according to the operation of the user. When the user sets the output power as “high”, the firmware 143 controls the pulse generator 141 to output the pulse with the wider pulse width to the IGBT 13. When the user sets the output power as “weak”, the firmware 143 controls the pulse generator 141 to output the pulse with the narrower pulse width to the IGBT 13.
When the induction heating cooker starts, the pulse generator 141 firstly outputs a first pulse to the IGBT 13 to turn on the IGBT 13. Because two input terminals of the comparator 142 are respectively coupled to two terminals of the inductive coil 10, the inductive coil 10 and the capacitor 11 start to oscillate after the IGBT 13 turns on and then off. Thus, the output of the comparator 142 is changed from the original negative saturation voltage to the positive saturation voltage. Each time when the pulse generator 141 receives the pulse, which is outputted from the comparator 142 and has the positive saturation voltage, it again outputs the pulse to the IGBT 13.
In addition, the firmware 143 also provides the protecting function. When the load pan is removed from the induction heating cooker, the energy of the inductive coil 10 cannot be released so that the voltage at the node A of the IGBT 13 is too high. Thus, the width of the outputted pulse has to be reduced. When the firmware 143 detects the too high voltage at the node A, it controls the pulse generator 141 to reduce the width of the outputted pulse. In addition, when the firmware 143 has detected a sudden rise of the voltage at the node B or C of the bridge rectifier 12, it represents that a surge occurs. At this time, the firmware 143 controls the pulse generator 141 to stop outputting pulses.
However, the firmware 143 needs a predetermined period of judging time, during which the energy caused by the inductive coil 10 cannot be released so that the voltage at the node A continuously rises. When the voltage drop of the IGBT 13 exceeds a predetermined voltage, such as 1200 Volts typically, the induction heating cooker may burn out.